Random Phantom Sound - Can't Find Source

I was using a self-built machine running Windows 7. Suddenly, about 3 months ago, random USB device disconnect-like sounds started occurring (sometimes 3x in a row, other times not for several hours) - I disabled all system sounds and it still continued, even though I didn't notice any USB hardware blips). Two months ago, I bought the new Intel NUC7i7BNH, installed Windows 10, and within a few hours the issue suddenly started all over again.
I've recorded the sound. Here's the link: https://soundcloud.com/finvarra/nuc-sound
I have replaced or swapped out literally all my hardware, disabled all Windows power-saving settings, turned off ALL Windows system sounds, tested the same configuration on 3 different computers (1 running Win 10, the others Win 7), been on several computer help forums, pretty much eliminated every computer-related fix Google (and several computer advisers) could suggest including testing all USBs and running USBDeview (with no hits) - and the same "disconnect" sound (or whatever it is) continues.
It's definitely coming through the speakers b/c I can lower the volume. I get no hardware hiccops. The randomness is what is really frustrating. For instance it was silent all yesterday morning, then 3 times in a half-hour, then nothing until this morning - just once - and then suddenly nearly a dozen times in 10 minutes. There is just no pattern - no particular change in electrical usage - and the sound cannot be controlled or even disabled in Windows. The events viewer never shows anything that corresponds to the sounds.
One hypothesis was that it could be electrical in nature. I tested all the sockets on the circuit (tested fine) and even replaced them. No change. I thought that maybe it could be a firewall issue in my router (notifications???) and disabled it. No change. Someone thought it might be an alert from the router and sent packets to the router to try to replicate the sound. Nothing.
To summarize... I'm looking for something that (a) sounds like the above; (b) comes through the speakers ; (c) is totally independent of anything I may be doing (or not doing anything) at the time; (d) and is completely random.
The NUC itself is running great... and I would just like to figure out WHAT the sound is, WHERE it's coming from, and HOW to stop it.
*I would appreciate ANY direction as this has been very frustrating. Thanks!